Millenial or Millennial: How To spell?
The word Millenial is misspelled against Millennial, an adjective meaning "pertaining to the millennium, to a thousand years; as, a millennial period; millennial happiness."
Millennial has double "nn" is mispronounced as "n"Remember double 'NN' in between the strings mille & ial which should not be misspelled with single 'N'
Wrong Spelling:  mille-n-ial  Correct Spelling:  mille-nn-ial 
Find more wrongly spelled words of Millennial to spell it well.